Recently, according to the progress in FTTH (Fiber to The Home), it is required to improve a micro-bend resistant property of an optical fiber in order to reduce transmission loss in the optical fiber.
The transmission loss in the optical fiber is increased by various kinds of external stress and micro-bend generated thereby. In order to protect the optical fiber from the external stress, generally in the optical fiber, a glass optical fiber is provided with a resin coating having a two-layers structure. Here, the optical fiber in which the glass optical fiber is provided with any coating is called an optical fiber.
There is disclosed an optical fiber, as the optical fiber having a coating layer of a two-layer structure, in which a water extractable rate of coating resin of the optical fiber is not larger than 1.5 mass·% after the optical fiber has been immersed in hot water of 60° C. for 68 hours (refer to patent literature 1).
Further, there is disclosed an optical fiber having a coated layer in which saturation charge voltage of the coated layer of the optical fiber is 0.2 kV to 0.7 kV and also a weight change rate thereof in a hot water immersion test at 60° C. is smaller than 3 weight·% to a weight before the immersion (refer to patent literature 2). Moreover, there is disclosed that, in a colored optical fiber having a colored layer, the colored layer is formed by a resin having a water extractable rate which is not larger than 3·% after having been immersed in hot water of 60° C. for 30 days, and a Young's modulus of the first coating layer in the colored optical fiber is not smaller than 0.5 MPa and not larger than 10 MPa (refer to patent literature 3).